Hot melt compositions find use as adhesives, coatings and sealants. These compositions do not, however, have good flame retarding characteristics in and of themselves, making it necessary to add flame retarding compounds which function under heat to yield compositions which are more difficult to ignite. These compounds act under intense heat and high temperatures by absorbing heat, by decomposing to form a nonflammable ash or coating to provide a barrier against heat transfer or by evolving products such as water vapor which retards or stops further propagation of a fire. Flame retardancy is measured by the oxygen index which is a vertical burning test. An oxygen index of 21 indicates that a composition would burn in atmosphere so the oxygen index is generally required to be 23 or higher to have flame retarding characteristics. The smoke generation of these materials must also be acceptably low before they will be approved for use by the Underwriters' Laboratory (UL).
Both organic and inorganic compounds have been used for flame retardancy. Organic flame retardants include chlorinated paraffins, phosphate esters, chlorinated alicyclic hydrocarbons, fluoropolymers, polyvinyl chloride and a variety of other halogenated organic compounds. Halogenated compounds produce products having poor thermal stability resulting in poor color retention and production of noxious fumes. In U.S. Pat. No. 4,169,082 to Kusterer, Jr. issued Sep. 25, 1979, a chlorinated paraffin wax is used as the flame retardant. Such halogenated compounds have been found to produce too much smoke in the compositions of the present invention. The disadvantages of using halogenated compounds are described in U.S. Pat. No. 5,378,856 to Allen issued Jan. 3, 1995. Usually these compounds are used in conjunction with an inorganic flame retardant such as antimony oxide.
Inorganic compounds, also referred to as fillers, include antimony oxide, magnesium hydroxide, zinc borate, barium metaborate, alumina trihydrate, zinc oxide, ammonium octamolybdate and magnesium distearate.
Polymeric based hot melt compositions containing flame retardant materials have been used for many purposes including sealing and jacketing of power cables, insulation and jacketing of electrical conductors and coatings for electrical and optical wires wherein the polymeric composition acts as a layer of insulation. The compositions typically used in these areas have a polymeric material and a filler. These types of compositions may be found in U.S. Pat. No. 4,849,135 to Reitz issued Jul. 18, 1989; U.S. Pat. No. 4,948,669 to Rolland issued Aug. 14, 1990; U.S. Pat. Nos. 4,575,184 and 5,059,651 to Ueno issued Mar. 11, 1986 and Oct. 22, 1991, consecutively; U.S. Pat. No. 5,378,856 to Allen issued Jan. 3, 1995 and U.S. Pat. No. 5,482,990 to Jow et al. issued Jan. 9, 1996.
U.S. Pat. No. 5,059,651 to Ueno specifically teaches a flame retardant and smoke suppressed polymeric composition comprising a radiation cured copolymer of ethylene and vinyl acetate containing 50-85% by weight vinyl acetate and a flame retarding and smoke suppressing amount of a filler mixture. These compositions contain uncrosslinked vinyl acetate polymers which gain strength only upon curing. At column 3 lines 37 to 43 it is stated that "a copolymer of ethylene and vinyl acetate containing about 60% by weight of vinyl acetate and having a molecular weight of about 200,000, as admixed with the same weight of a finely divided inorganic filler (aluminum hydroxide), has a tensile strength as low as 0.1 kg/mm.sup.2, and is of no practical use." Furthermore, these compositions are very high in viscosity and are intended to be used as sheathing for electrical wire.
U.S. Pat. No. 5,317,051 to Harashige et al. issued May 31, 1994 teaches a flame-retardant olefin polymer composition superior in surface whitening prevention having a blend of a resin component consisting principally of an olefin resin and an olefin polymer modified with an unsaturated carboxylic acid or derivative thereof, a flame retardant and a whitening preventing agent. Harashige teaches that these compositions are useful for electric wires, cables, packages, sealing materials, hoses, films, molded products and master batches.
In addition to the above applications, flame retardant compositions are useful in the air filter industry, and in particular are useful for High Efficiency Particulate Air (HEPA) Filters. In this situation, the compositions act as adhesives to adhere the pleats of the filters together. These adhesives are extruded onto the filters and each filter pleat is adhered to the next. The difficulty in using the polymeric compositions described above is that the viscosities of these compositions are too high for use in hot melt application equipment. In order to lower the viscosity of a hot melt adhesive, it is common to replace part of the polymer with lower molecular weight materials such as resin and wax. However, when commonly used proportions of these materials are used, the smoke generation upon burning is unacceptably high. In addition, lowering the polymer content to decrease viscosity can result in decreased filler acceptability and decreased flexibility of the resultant composition.
HEPA filters utilizing a hot melt adhesive as a filter pleating adhesive are discussed in U.S. Pat. No. 4,365,980 to Culbert et al. issued Dec. 28, 1982. Culbert discusses utilizing a hot melt adhesive, "Instant Lok," supplied by National Adhesives, Inc. This hot melt is ethylene vinyl acetate based and is generally used for case and carton sealing. It has greater than 25% tackifying resin, has no flame retardant and is therefore not a flame retardant adhesive. See column 3 lines 63 to 68.
It is an object of the present invention to provide a novel polymeric based flame retardant composition which has a relatively low viscosity, is flexible in a thin bead or film and adheres well to a variety of porous substrates while maintaining a high oxygen index and low smoke generation.